1. Field of the Invention
The present invention relates to an active damping oscillator suitably usable for an active vibration damping device which is attached to a vibrating member or a member of a vibration transmitting system, for providing an active damping effect with respect to a vibration of such a vibrating member or an object associated with such a vibration transmitting system.
2. Discussion of the Related Art
For damping a vibration of a vibrating member, there have been generally employed various vibration dampers such as shock absorbers, vibration damping means which utilizes a damping effect exhibited by rubber or other elastic materials, and vibration isolating or insulating means which utilizes a spring effect exhibited by coil springs or rubber or other elastic materials. These vibration dampers are all adapted to provide a passive vibration damping or isolating effect, and are not capable of sufficiently damping or isolating a vibration whose characteristics tend to vary. In the light of this drawback of the known vibration dampers, there have recently been developed active vibration damping devices adapted to oscillate a vibrating member or a member of a vibration transmitting system, for actively or positively damping a vibration whose characteristics tend to vary. Example of such active vibration damping devices are disclosed in JP-A-61-220925 and JP-A-64-83742.
The active vibration damping device requires an active damping oscillator for generating an oscillating force or oscillatory movements. In the active damping oscillator, the frequency of the generated oscillation is required to be easily and accurately controlled. To meet this requirement, the active damping oscillator as disclosed in JP-A-6-235438 includes: a first and a second support member which are connected to each other by a rubber member, a metal spring or any other elastic member, such that the first and second support members are displaceable relative to each other; a permanent magnet which cooperates with one of the first and second support members to form a closed magnetic path; and a coil movably disposed in a magnetic gap in the closed magnetic path and fixed to the other of the first and second support members, so that the first and second support members are oscillated relative to each other by an oscillating force generated upon energization of the coil, namely, by a Lorentz force (electromagnetic force) generated by the energized coil.
In the active damping oscillator described above, the first and second support members connected to each other by the elastic member constitute a vibration system having a certain natural frequency (undamped natural frequency or frequency of vibration of the normal mode), so that the waveform of a control signal or electric current to be applied to the coil and the waveform of the generated oscillation are not linear, and have a high rate of change in the phase, in a frequency band near the natural frequency of the vibration system, in particular. Accordingly, the active damping oscillator suffers from difficult control of the generated oscillation.
U.S. Pat. No. 5,231,336 and U.S. Pat. No. 5,718,418 disclose an active damping oscillators wherein the first and second support members, which are not connected to each other by an elastic member, are positioned and displaced relative to each other by a magnetic force.
In the active damping oscillators described just above, an oscillating force is based on a Lorentz force (electromagentic force) generated upon energization of a coil disposed in a magnetic field. Accordingly, these active damping oscillators suffer are less likely to be capable of efficiently generating an oscillating force with high stability. Described more specifically, the oscillator disclosed in U.S. Pat. No. 5,231,336 is arranged such that the coil is disposed in an open magnetic path which is formed by magnetic poles which are not opposed to each other. In this arrangement, the magnetic flux density and the efficiency of generation of the Lorentz force by energization of the coil are low. In the oscillator disclosed in U.S. Pat. No. 5,718,418, the coil is disposed in a magnetic gap formed between two opposed magnetic poles. However, this oscillator has a problem relating to a structure for supporting the coil. That is, the coil supporting structure does not have a sufficiently large mechanical strength, resulting in difficulty to assure high degrees of operating stability and durability of the oscillator. If the number of windings of the coil is increased for the purpose of increasing the oscillating force, the magnetic gap is inevitably enlarged, causing a decrease in the magnetic flux density, resulting in difficulty to generate a sufficiently large amount of oscillating force.